Momentum compensated rotational actuators are required to rotationally articulate precision pointed devices in a pointing disturbance rich environment with constraints on the pointing control momentum magnitude imparted to the supporting structure. Many momentum compensation devices use mechanical mechanisms such as the differential gear with means to minimize mechanical backlash to maintain the ratio of the rotational rate of the planetary carrier relative to the rotational rate of the pointed device a negative constant.
Precision pointing also requires precise control of the net torque from all sources acting on the pointed device, and currently there are no means to accurately measure either pointed device control torques or disturbance torques. Pointed device control torque is generally determined as a function of observed inertial states such as angular acceleration, angular rate, and angular position of the base structure, the pointed device, or both. Disturbance torques acting on the pointed device over most of the operational range are generally a continuous well behaved function of the pointed device angular rate relative to its bearing support structure and have been adequately controlled over a restricted operational range for most precision pointing applications. However, when the pointed device angular rate relative to the bearing support structure passes through zero, large, extremely non-linear friction torques act on the pointed device through the rotational support bearings making disturbance torque prediction, control, and precision pointing in this region impossible. Accordingly, it is the object of this invention to provide a means to eliminate the non linear friction torque effects over a large operational angular rate range on a precision pointed device rotationally driven relative to the base vehicle by a momentum compensated, differential gear, rotary actuator.